Use a hash. Here's code to do both and more. It assumes that each element is unique in a given array: 
 
        @union = @intersection = @difference = (); 
        %count = (); 
        foreach $element (@array1, @array2) { $count{$element}++ } 
        foreach $element (keys %count) { 
                push @union, $element; 
                push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; 
                } 
 
Note that this is the symmetric difference, that is, all elements in either A or in B but not in both. Think of it as an xor operation.

#!/usr/bin/perl 
use strict; 
use warnings; 
 
my ($word1, $word2) = qw/ abcd abcdxyz /; 
 
my %letters; 
 
$letters{$_} .= 'a' for split //, $word1; 
$letters{$_} .= 'b' for split //, $word2; 
 
my @uncomman; 
 
for my $ltr (keys %letters) { 
    push @uncomman, $ltr if $letters{$ltr} =~ /^b/ || $letters{$ltr} =~ /a$/; 
} 
 
print @uncomman;